This application is based on Patent Application No. 2001-143572 filed May 14, 2001 in Japan, the content of which is incorporated hereinto by reference.
1. Field of the Invention
The present invention relates to a silica-based optical waveguide circuit and a fabrication method thereof, and more specifically, to a silica-based optical waveguide that serves to reduce possible coupling losses when coupled to another optical waveguide or fiber as well as a fabrication method thereof.
2. Description of the Related Art
With the rapid development of optical communications, there have been growing demands for increases in scale and integration level of silica-based optical waveguides, a decrease in manufacture costs thereof, and improvement of mass productivity thereof. To increase the scale and integration level, the confinement of light in a core must be enhanced. This serves to reduce the distance between adjacent optical waveguides and the bending radius thereof, thereby allowing a large number of optical waveguides to be arranged in a limited area.
When the confinement of light in a silica-based optical waveguide is enhanced, a possible coupling loss increases when this waveguide is coupled to an optical fiber or another optical component. A number of methods have been known for reducing this coupling loss. For example, a method of increasing the size of that portion of the core which correspond to an input and output section of the waveguide so as to accommodate the spot size of the fiber is practical in that fabrication tolerance or fiber connection tolerance can be increased. By way of example, Japanese Patent Application Laid-open No. 2001-56415 describes a spot size converter using a vertical and horizontal tapered waveguide.
FIG. 7 shows a conventional spot size converter. The spot size converter has a lower cladding layer 52 formed on a substrate 51 and having a waveguide core 53a formed thereon. The waveguide core 53a is further covered with an upper cladding layer 54. Such a spot size converter has a simple structure and is effective in reducing the coupling loss. For example, for waveguide core specifications with which a coupling loss corresponding to about 2 dB may be caused by a difference in spot size between the waveguide core and the fiber, the coupling loss can be reduced to 0.2 dB or less by increasing the spot size using the vertical and horizontal taper.
However, an optical waveguide having such a spot size converter has a larger core thickness at the input and output section thereof than ordinary optical waveguides, thereby requiring a longer time to process the core by reactive ion etching. That is, such a process may reduce mass productivity compared to ordinary processes.
It is an object of the present invention to provide a silica-based optical waveguide that serves to reduce the time required for production while allowing a spot size converting function to work sufficiently as well as a fabrication method therefor.
To attain this object, the present invention provides a silica-based optical waveguide circuit comprising a substrate, a lower cladding layer placed on the substrate, a waveguide core for transmitting light formed on the lower cladding layer, an input/output waveguide core whose thickness H is thicker than the waveguide core, a tapered portion for connecting the input/output waveguide core and the waveguide core, and an upper cladding layer that covers the lower cladding layer and the cores, the waveguide circuit further comprises a core layer at each side of the input/output waveguide core, and a thickness T of the core layer at each side of the input/output waveguide core is smaller than the thickness H.
With this configuration, the core layer thinner than the input/output waveguide core is formed at each side of the input/output waveguide core, thereby reducing the time required to produce a silica-based optical waveguide circuit while allowing a spot size converting function to work sufficiently.
The above and other objects, effects, features and advantages of the present invention will become more apparent from the following description of embodiments thereof taken in conjunction with the accompanying drawings.